Join Dependency

A Join Dependency (JD) over a relation schema R is a statement of the form |><|[schema(R₁), schema(R₂), …, schema(R_n)], where R = {R₁, R₂, …, R_n} is a database schema such that schema(R)=schema(R).

 

A JD |><|[R] is satisfied in a relation r over R, denoted r |= |><|[R], if   r = π _schema(R1) (r) |><| π _schema(R2) (r) |><| … π _schema(Rn) (r)

A JD  in our banking example

Define a new relation schema

Loan-info-schema = (branch-name, customer-name, loan-number, amount) in this banking example.

We can define a relation loan-info(Loan-info-schema) as the set of all tuples on Loan-info-schema such that:

• The loan represented by loan-number is made by the branch named branch-name.
• The loan represented by loan-number is made to the customer named customer-name.
• The loan represented by loan-number is in the amount given by amount.

 

The preceding definition of the loan-info relation is a conjunction of three predicates: one on loan-number and branch-name, one on loan-number and customer-name, and one on loan-number and amount.

 

It can be shown that the preceding intuitive definition of loan-info logically implies the join dependency |X| [ {loan-number, branch-name}, {loan-number, customer-name}, {loan-number, amount} ]